1. Field of the Invention
This invention relates to measurement of the liquid flow velocities and liquid mass flowrates in a multiphase flow containing at least two liquid phases. As used herein the term "phase" is intended to refer to separate immiscible liquid phases such as oil and water as well as liquid and gas phases. As used herein the term "multiphase" is intended to refer to a mixture including at least two such phases.
Determination of mass flowrate of components in a flow requires a knowledge of the respective flow velocity of each phase. As used herein the term "velocity" is intended to refer to mean flow velocity in the flow direction, for example along a tube. Hitherto the accurate measurement of flow velocity of liquid phases in a flow has proved difficult and has hampered the development of mass flowrate measurement techniques.
2. Description of the Prior Art
Measurement of the mass flowrates of components of flows containing several phases is desirable in many fields but is of particular importance in the oil industry.
Crude oil production is normally accompanied by gaseous hydrocarbons and water. These three components are piped from the oil well as a multiphase mixture. The mass flow rate measurement of the oil, water and gas from individual oil wells is important for better reservoir management, better production allocation, and optimisation of total oil production over the field life. Normally, the required accuracy of determination of mass flow of each component is 5%.
Additionally, there is often a need to measure the relative concentrations of oil and water in a flow after separation of the gas and some of the water. This measurement can present considerable practical difficulty particularly where the densities of the oil and water are the same or similar.
Current practice for the measurement of mass flowrate of the components of oil well flows is to periodically physically divert the well output to a test separator. After separation the flow rate of each component is measured with conventional devices such as orifice or turbine flow meters. There are several inherent disadvantages associated with this technique. Firstly, accurate measurement requires stabilised well flow which can take some time to establish. Often testing the output of a single well may take a whole day. In addition, the physical size of the separator and associated equipment occupies significant space which can lead to increased costs on off-shore platforms. Finally, in practice it is not feasible to provide each well with its own test separator system and often many wells share a common facility. Continuous monitoring of the output of each well is therefore not possible.
Various techniques have been suggested for on-line mass flow measurement of multiphase mixtures. Most depend on determination of the concentration of one or more of the components coupled with a determination of either the mean velocity of one or more of the components or the total mass flow of the mixture. Concentration measurement by capacitance is described in a paper entitled "On-line measurement of oil/water/gas mixtures using a capacitance sensor" by Beck M. S. Green R. G., Hammer E. A. and Thorn R, Measurement 3 (1) 7-14 (1985). Measurement of component concentration using a dual energy gamma-ray transmission technique has also been described by the following:
Fanger U., Pepelnik R. and Michaelis W.--Determination of conveyor-flow parameters by gamma-ray transmission analysis, pp. 539-550 in Nuclear Techniques and Mineral Resources 1977, IAEA, Vienna, 1977.
Michaelis W. and Fanger H. U.--Device for determining the proportions by volume of a multiple-component mixture, U.K. Patent Application GB2083908 A, 1982.
Abouelwafa M.S.A. and Kendall E.J.M.--The measurement of component ratios in multiphase systems using gamma-ray attenuation, J.Phys.E.: Sci. Instrum, 131 341-345 (1980).
Kendell E.J.M.--Gamma-ray analysis of multicomponent material, U.K. Patent Application GB 2088050 A, 1982.
Tomada T., Komaru M., Badono S., Tsumagari K. and Exall D.--Development of gamma-ray oil/water/gas fraction meter for crude oil production systems, Paper presented at the International Conference on Industrial Flow Measurement On-shore and Off-shore, 22-23/9/87, London.
Microwave measurement of component concentration is also known from U.S. Pat. No. 4,301,400. Neutron inelastic scatter techniques have also been used.
Velocity is usually determined by measuring the time taken for the fluid to flow between two sensors, one downstream of the other, by cross-correlation of the output signals of the two sensors, and combining this with the distance between the two sensors. This technique is described in Beck M. S. and Plaskowski A.--Cross Correlation Flowmeters, Adam Hilger, Bristol, 1987. Doppler measurements are an alternative technique to determine velocity and have been described by Stuchly S. S., Sabir M. S. and Hamid A.--Advances in monitoring of velocities and densities of particulates using microwave Doppler effect, IEEE Trans. Instrumentation and Measurement IM-26 (1) 21-24. The mass flow of a two component mixture such as oil and water can be determined by a gauge depending on Coriolis force. This is described by Liu K. T. and Revus D. E.--Net-oil computer improves water-cut determination, Oil and Gas Journal pp. 41-45, 19 Dec. 1988.None of these techniques fully solves the requirement for accurate on-line determination of mass flow rate of each separate component of a multiphase flow.